1. Field of the Invention
This invention relates to novel electrically conductive forms of poly(substituted heterocycles), especially poly(substituted thiophenes), and to solutions comprising such polymers either in the conductive form or non-conductive form. Another aspect of this invention relates to a method of using the solution of this invention to form conducting polymer articles, including films, fibers, and coatings and methods of using such solutions as conducting liquids.
2. Prior Art
There has recently been an increased interest in the electrical conductivity of polymeric systems. For example, U.S. Pat. Nos. 4,321,114 and 4,442,187 are directed to conjugated polymers having conjugation in all or a part of at least one backbone chain thereof, such as polyacetylene, polyphenylene, and poly(phenylene sulfide). It has recently been discovered that these conjugated backbone polymers can be chemically doped in a controlled manner with electron acceptor and/or electron donor dopants to produce electrically conducting polymers. Doping procedures and certain representative doped polymers are described in U.S. Pat. Nos. 4,222,903 and 4,204,216.
In the general field of conducting polymers, it is believed very difficult to dope one of these conjugated backbone polymers to the extent that it becomes a good conductor (.about.10.sup.-3 -100 ohm.sup.-1 cm.sup.1) and thereafter dissolve the polymer in any solvent-system. U.S Pat. No. 4,452,727 discloses novel polymer solutions containing a doped sulfur-containing or oxygen-containing aromatic polymer. The solvent of this solution is restricted to Lewis Acid halides having a liquid phase under atmospheric pressure for at least one temperature between -150.degree. C. and +100.degree. C., such as arsenic trifluoride, phosphorus trifluoride, phosphorous pentafluoride, phosphorus trichloride, boron trifluoride and the like. These solutions can be used to form articles, as for example, by casting the solution onto a substrate, and removing the solvent. This solution and method represents a significant advancement over the art; however, it does suffer from certain economic and practical disadvantages resulting from the cost and high environmental reactivity and toxicity of the specific solvents which must be used and the environmental instability of the doped, cast conductive polymer.
A few conductive species of polyalkylthiophenes are known, having been primarily prepared by electrochemical polymerization. Illustrative of such species are poly(3-methylthiophene) and poly(3,4-dimethylthiophene). R. J. Waltman, J. Bargon, and A. F. Diaz, J. Phys. Chem., 1983, 87, 1459-1463. G. Tourillon, D. Govrier, P. Garnier, and D. Viven, J. Phys. Chem., 1984 88, 1049-1051. S. Hotta, T. Hosaka, and W. Shimotsuma, Syn. Metals. 1983, 6, 317-318. However, the polymers, prepared electrochemically, are not soluble in solvents such as acetonitrile, propylene carbonate, tetrahydrofuran, dichloromethane, dimethyl formamide, nitrobenzene, nitropropane, toluene, and the like. In the absence of solutions, or plasticized forms, the ability to economically form certain articles out of the conducting forms of these poly(alkylthiophenes), especially semi-conducting and conducting polymer films, fibers, and coatings, is greatly restricted. In fact, the electrochemical methods are reported to give homogeneous polymer films only up to film thickness of about 2000 .ANG.. Powdery deposits are obtained when attempts are made to grow films thicker than this. (G. Tourillon and F. Garnier, J. Poly. Sci. Poly. Phys. Ed., 1984, 22, 33-39.)
The unsubstituted polythiophenes form highly conductive complexes on doping which are not stable in normal environments (containing air or water vapor). However, electrochemically prepared conductive poly(3-methylthiophene) is environmentally stable. (G. Tourillon and F. Garnier, J. Electrochem. Soc., Electrochem. Sci, Techn. 1983, 130, 2042-3.
U.S. Pat. No. 4,521,589 describes linear poly(3-alkyl-2,5-thienylene) polymers having from 20 to 100 repeat units which can be used to provide a semiconductor. However, polymers of this sort with only 100 repeat units do not form articles or films with mechanical properties which are generally useful. Although improved solubility is claimed, this patent teaches by example that higher molecular weight fractions become more insoluble. Clearly, improvements are needed and desired to prepare processible high molecular weight and highly conductive polymer compositions which would be of general applicability.